Estimate Peak Runoff With the Rational Method
Use the Rational Method to estimate peak runoff for a small drainage area and explain its input assumptions.
Estimate peak runoff for a 1.8 acre paved-and-landscaped area with runoff coefficient C = 0.78 and rainfall intensity i = 3.5 in/hr. Rational Method: Q = C i A for small drainage areas when inputs match the site and design storm. The shortcut is to multiply numbers without checking whether the coefficient, intensity, and area describe the same drainage condition. Step 1 Write the inputs with units: C = 0.78, i = 3.5 in/hr, A = 1.8 ac. Labeling units catches most beginner mistakes. Step 2 Apply the Rational Method relationship: Q = C x i x A. In U.S.…
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